Mobile phones, wireless LANs, and other wireless communication devices are provided with a high frequency circuit, and a power amplifier is used in the high frequency circuit. A power amplifier is a component required in a transmitting circuit of a communication device. In recent years in particular, it is expected that a wireless LAN function is mounted in a mobile phone, and there is a need for smaller-sized high frequency circuits. In view of this need, a high frequency module in which a power amplifier and a filter disposed in the pre- and post-stages are integrally formed in the high frequency circuit.
FIG. 5 is a block diagram showing an example of the configuration of a high frequency module.
As shown in FIG. 5, a high frequency module 2 is composed of a power amplifier 11, a first filler 12 disposed in the pre-stage of the power amplifier 11, and a second filter 13 disposed in the post-stage of the power amplifier 11. An input end of the high frequency module 2 is connected to a transceiver IC (RFIC) of the high frequency circuit, and an output end is connected to an antenna 52 by way of an antenna switch 51. The first filter 12 is a band-pass filter (BPF) for removing spurious signals generated in a mixer in the RFIC, and the second filter 13 is a low-pass filter (LPF) or a band-pass filter for suppressing spurious signals of a multiplication wave generated in the power amplifier 11 (see Japanese Laid-open Patent Application No. 2005-101893).
Since the RFIC disposed in the pre-stage of the high frequency module 2 has a balanced output, the high frequency module 2 must also be provided with balanced input. In a conventional high frequency module 2, a balance to unbalance conversion is carried out using a balun and a connection is made to the power amplifier 11 by way of the first filter 12, which is a band-pass filter. However, in recent years, a so-called balance filter having a balun function is often used (see Japanese Laid-open Patent Application No. 2005-45447).
FIG. 6 is a schematic cross-sectional view showing a conventional structure of the high frequency module 2.
As shown in FIG. 6, the high frequency module 2 is provided with a multilayered substrate 10, a power amplifier IC 11 mounted on an upper surface of the multilayered substrate 10, first and second filters 12, 13 formed on an inner layer of the multilayered substrate 10. Thermal vias 22 are disposed directly below the power amplifier IC 11, and the thermal vias 22 are formed completely through the multilayered substrate 10 in the vertical direction and are connected to the power amplifier IC 11 and the ground terminals 20 on the bottom surface of the substrate (see Japanese Laid-open Patent Application Nos. 2006-121147 and 2005-123909).
Ground patterns 23, 24 are disposed on an inner layer of the multi layered substrate 10, and the ground patterns 23, 24 are connected to the thermal vias 22. The input end of the first filter 12 of the inner layer is connected to the high frequency module input terminal 18 by way of a via hole, and the output end of the second filter 13 is connected to a high frequency module output terminal 19 by way of a via hole.
Examples of other prior arts include a structure for separating electromagnetic coupling between different bands or the connection between transceiver circuits using ground vias and patterns (see Japanese Laid-open Patent Application Nos. 2006-140862, 2004-235877, 2005-244336), a structure in which interference-preventing ground parts are disposed between an surface acoustic wave device (filter) and the power amplifier mounted on the surface of a dielectric substrate (see Japanese Laid-open Patent Application No. 2005-244336). Also known is a large-scale high frequency module in which the RF antenna switch circuit and a plurality of diplexers are integrated (see Japanese Laid-open Patent Application No. 2006-157880). Also known is a dipole resonance filter having two inductor electrodes or a tripole resonance filter having three inductor electrodes as a filter incorporated into a nigh frequency module (see Japanese Laid-open Patent Application No. 2007-235435).
As described above, thermal vias for dissipating heat are required because the power consumption of a power amplifier is considerable and the amount of generated heat is also considerable. Conventionally, other circuits and wiring cannot be disposed directly below the power amplifier IC 11 and there is a negative effect in relation to reducing the size of the high frequency module because the thermal vias 22 are disposed in a position facing the lower surface of the power amplifier IC 11. Also, it is preferred that the wiring of each filter be shortened as much as possible in order to prevent degradation of insertion loss, and it is also preferred that each filter be disposed in the vicinity of the power amplifier, but the presence of thermal vias prevents such efforts.
On the other hand, there is a problem in that the spacing between the first filter and second filter disposed in the pre-stage and post-stage of the power amplifier are brought closer together as the high frequency module is made smaller, the filters become electromagnetically coupled, and isolation is reduced. When isolation between the filters is low in comparison with the gain of the power amplifier, there is a problem in that feedback is applied to the power amplifier by way of the pre- and post-filters, the operation of the power amplifier becomes unstable, and in the worst case, the power amplifier oscillates.